1. Field of the Invention
The present invention relates to a radiographic intensifying screen, and more particularly, to a radiographic intensifying screen employing a phosphor containing iodine.
2. Description of the Prior Art
In a variety of radiography such as medical radiography for diagnosis and industrial radiography for nondestructive inspection, a radiographic intensifying screen is generally employed in close contact with one or both surfaces of a radiographic film such as an X-ray film.
The radiographic intensifying screen has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent protective film of a polymer material is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock. The phosphor layer comprises a binder and phosphor particles dispersed therein. When excited with a radiation such as X-rays having passed through an object, the phosphor particles emit light of high luminance in proportion to the dose of the radiation. Accordingly, the radiographic film placed in close contact with the phosphor layer of the intensifying screen can be exposed sufficiently to form a radiation image of the object, even if the radiation is applied to the object at a relatively small dose.
It is desired that the radiographic intensifying screen has a high radiographic speed and provide an image of high quality (high sharpness, high graininess, etc.). Especially when the object is a human body, the radiographic speed of the intensifying screen is desired to be as high as possible to reduce the radiation dose applied to the object.
As a phosphor employed in the radiographic intensifying screen, there has been proposed phosphors containing iodine as a component thereof such as a divalent europium activated barium fluorohalide phosphor (BaFX:Eu.sup.2+, in which X is I or a combination of I and Cl and/or Br), cesium iodide phosphors (CsI:Na, CsI:Tl), and a terbium and/or thulium activated rare earth oxyhalide phosphor (LnOX:Tb,Tm, in which Ln is at least one rare earth element selected from the group consisting of La, Y, Gd and Lu; X is I or a combination of I and F and/or Cl and/or Br).
However, when these phosphors containing iodine are used in the radiographic intensifying screen, the phosphor layer is liable to be colored yellow gradually because the iodine is liberated from the phosphor to produce an iodine molecule (I.sub.2). Such yellowing of the phosphor layer causes a problem of prominently decreased radiographic speed in the intensifying screen.